docs: def show_preview(viewer_path, pdf_file_name):
      """Run the PDF viewer at the path viewer_path on the file pdf_file_name."""
      try:
          cmd = [viewer_path, pdf_file_name]
          run_external_subprocess_in_background(cmd)
      except (subprocess.CalledProcessError, OSError, IOError) as e:
          print("\nWarning from pdfCropMargins: The argument to the '--viewer' option:"
                "\n   ", viewer_path, "\nwas not found or failed to execute correctly.\n",
                file=sys.stderr)
      return